Functionalized metal chalcogenides

ABSTRACT

The present disclosure relates to a composition that includes a metal chalcogenide having a surface and a ligand, where the ligand is covalently bound to the surface. In some embodiments of the present disclosure, the metal chalcogenide may be defined by MXz, where Z is between 1 and 3, inclusively, M (a metal) includes at least one of Sc, Zr, Hf, Zr, Ti, Nb, Ta, V, Mo, Cr, Re, W, S, Pt, Fe, Cu, Sb, In, Zn, Cd, P, and/or Mn, and X (a chalcogenide) includes at least one of S, Se, and/or Te.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application No. 62/595,828, filed Dec. 7, 2017, the disclosure of which is incorporated herein by reference in its entirety.

CONTRACTUAL ORIGIN

The United States Government has rights in this disclosure under Contract No. DE-AC36-08GO028308 between the United States Department of Energy and Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory.

BACKGROUND

Electrochemical reduction of protons to molecular hydrogen (H₂) is a carbon-free energy conversion technology that, with increased research and development, could be a front-runner for renewable fuels. Currently, the most efficient catalyst (platinum) for H₂ generation is too expensive and consequently is not produced on a large enough scale to be used as a global energy resource. Therefore, a lower cost catalyst with high efficiency is needed.

SUMMARY

An aspect of the present disclosure is a composition that includes a metal chalcogenide having a surface and a ligand, where the ligand is covalently bound to the surface. In some embodiments of the present disclosure, the metal chalcogenide may be defined by MXz, where Z is between 1 and 3, inclusively, M (a metal) includes at least one of Sc, Zr, Hf, Zr, Ti, Nb, Ta, V, Mo, Cr, Re, W, S, Pt, Fe, Cu, Sb, In, Zn, Cd, P, and/or Mn, and X (a chalcogenide) includes at least one of S, Se, and/or Te.

In some embodiments of the present disclosure, the metal chalcogenide may include at least one of WTe₂, WSe₂, WS₂, MoS₂, MoSe₂, and/or MoTe₂. In some embodiments of the present disclosure, the metal chalcogenide may include at least one of ScS₂, ScSe₂, SeTe₂, ZrS₂, ZrSe₂, HfS₂, HFSe₂, HfS₃, HfSe₃, ZrS₃, ZrSe₃, ZrTe₃, TiS₂, TiS₃, TiSe₃, NbS₂, NbSe₂, NbS₃, TaS₂, TaSe₂, TaS₃, TaSe₃, VS₂, VSe₂, MoReS₂, CrS₂, WSSe₂, MoSSe, MoWSe₂, MoTe₂, WTe₂, WS₂, MoS₂, MoSe₂, MoTe₂, ReS₂, ReSe₂, ReNbS₂, ReNbSe₂, PtS₂, PtSe₂, PtTe₂, FeSe, CuS, CuSbS₂, CulnS₂, CulnSe₂, ZnS, ZnSe, CdS, CdSe, FePS₃, FePSe₃, MnPS₃, MnPSe₃, CdPS₃, and/or CdPSe₃. In some embodiments of the present disclosure, metal chalcogenide may be in a form comprising at least one of a sheet and/or a particle. In some embodiments of the present disclosure, the sheet may include at least one monolayer of the metal chalcogenide. In some embodiments of the present disclosure, the sheet may have a thickness between 5.0 nm and 30 nm. In some embodiments of the present disclosure, the particle may have a characteristic length between 5.0 nm and 50,000 nm.

In some embodiments of the present disclosure, the ligand may have at least one of an electron donating functional group and/or an electron withdrawing functional group, as measured by at least one of a Hammett parameter and/or a workfunction. In some embodiments of the present disclosure, the ligand may include a moiety that includes at least one of a halogen, an amine, an amide, a ketone, thiol, and/or a nitro group. In some embodiments of the present disclosure, the ligand may further include an intermediate group that includes at least one of an aromatic group, a saturated hydrocarbon chain, and/or an unsaturated hydrocarbon chain. In some embodiments of the present disclosure, the aromatic group may be a phenyl ring (Ph). In some embodiments of the present disclosure, the ligand may include at least one of NO₂Ph, ClPh, BrPh, OCH₃Ph, and/or (CH₃CH₂)₂NPh, and the moiety may be in at least one of the ortho position, the meta position, and/or the para position of the phenyl ring. In some embodiments of the present disclosure, the ligand may include at least one of p-NO₂Ph, 3,5-Cl₂Ph, p-BrPh, p-OCH₃Ph, and/or p-(CH₃CH₂)₂NPh

In some embodiments of the present disclosure, the metal chalcogenide may be in a substantially crystalline phase. In some embodiments of the present disclosure, at least a portion of the crystalline phase may be a 1 T metallic phase. In some embodiments of the present disclosure, the Hammett parameter may be between −0.5 and 1.0, inclusively. In some embodiments of the present disclosure, the workfunction may be between 3.0 eV and 6.0 eV, inclusively. In some embodiments of the present disclosure, the composition may catalyze the hydrogen evolution reaction (HER), and the composition may generate a HER catalytic current density of at least 10 mA/cm² when provided with an overpotential of less than 1000 mV.

An aspect of the present disclosure is a composition that includes MoS₂ in a form of at least one of a particle and/or a sheet and a ligand covalently bound to the MoS₂, where the ligand includes at least one of p-NO₂Ph-, 3,5-Cl₂Ph-, p-BrPh-, p-OCH₃Ph-, and/or p-(CH₃CH₂)₂NPh-, the MoS₂ is it least partially in a 1 T crystalline phase and maintains the 1 T crystalline phase for at least two hours, the form has a characteristic length between 5.0 nm and 50,000 nm, inclusively, the composition is characterized by a Hammett parameter between −0.5 and 1.0, inclusively, the composition catalyzes the hydrogen evolution reaction (HER), and the composition generates a HER catalytic current density of about 10 mA/cm² when provided with an overpotential of less than 1000 mV.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than limiting.

FIG. 1A illustrates a schematic of the various ligands on metallic phase, a metal chalcogenide, for example MoS₂, according to some embodiments of the present disclosure.

Although the ligands in FIG. 1A are shown in the para position on the phenyl rings, ligands in at least one of the para position, the ortho position, and/or the meta position of a phenyl ring are considered within the scope of the present disclosure.

FIG. 1B illustrates an atomic force microscopy (AFM) image of a chemically exfoliated metallic MoS₂ nanosheets, according to some embodiments of the present disclosure.

FIG. 1C illustrates extracted line profiles that demonstrate the height and length of chemically exfoliated metallic MoS₂ nanosheets. A height distribution of 3 nm to 5 nm and a length distribution of 250 nm to 400 nm are observed from the AFM topographic image (FIG. 1B).

FIG. 2A illustrates Raman spectra of pristine and functionalized metallic phase MoS₂ along with bulk semiconducting phase MoS₂ as a comparison, according to some embodiments of the present disclosure. The spectra were taken with an excitation wavelength of 633 nm. All spectra contain the expected MoS₂ transitions: E¹ _(2g) (˜383 cm⁻¹) and A_(1g) (˜406 cm⁻¹). The metallic phase MoS₂ has the characteristic metallic peaks (J₁, J₂, and J₃ located at ˜154, ˜230, and ˜326 cm⁻¹, respectively). The functionalized MoS₂ spectra have the J₁ and J₂ peaks but do not have the J₃ peak.

FIG. 2B illustrates DRIFTS spectra of MoS₂ nanosheets, both functionalized and unfunctionalized metallic phase, according to some embodiments of the present disclosure.

FIGS. 3A and 3B illustrate XPS of (FIG. 3A) Mo 3d and (FIG. 3B) S 2p for unfunctionalized metallic (1 T) MoS₂ and functionalized metallic MoS₂, according to some embodiments of the present disclosure. Since the ligands attach to the S atom, the Mo 3d spectra are similar and the S 2p spectra differ between unfunctionalized and functionalized MoS₂. The Mo 3d spectra highlight that the MoS₂ is dominated by the metallic phase.

FIG. 3C illustrates the ratio of metallic Mo—S to S—C S 2p XPS peak areas as a function of Hammett parameter. The more electron withdrawing ligands (more positive Hammett parameter) have more ligands attached to the MoS₂ nanosheets than electron donating ligands (negative Hammett parameter)

FIG. 4 illustrates the relationship between workfunction and Hammett parameter, according to some embodiments of the present disclosure. The most electron donating ligand (Et₂NPh) has the shallowest workfunction. The Hammett parameter has a positive correlation to the workfunction and varies by 800 meV over the various ligands.

FIG. 5A illustrates linear sweep voltammograms (LSV) and FIG. 5B the corresponding Tafel plots for glassy carbon electrodes deposited with functionalized and unfunctionalized metallic MoS₂ as well as the unfunctionalized bulk semiconducting phase of MoS₂, according to some embodiments of the present disclosure. LSVs were taken at 5 mV/s in 0.5 M H₂SO₄ with a Ag/AgCl reference electrode and vitreous carbon counter electrode. Both panels have been iR corrected. Pt and glassy carbon electrodes for the hydrogen evolution reaction (HER) are included in FIG. 5A as a reference.

FIG. 6A illustrates the average overpotential (taken at current density of 10 mA/cm²) and FIG. 6B the Tafel slope for the MoS₂ functionalized electrodes, according to some embodiments of the present disclosure. Measurements were performed in 0.5 M H₂SO₄ with a Ag/AgCl reference electrode and a vitreous carbon counter electrode.

FIG. 7 illustrates the electrochemical impedance spectra (Nyquist plots) for functionalized metallic phase MoS₂, unfunctionalized metallic phase MoS₂, and unfunctionalized bulk semiconducting phase MoS₂ on glassy carbon electrode at −0.29 V vs RHE, according to some embodiments of the present disclosure.

FIGS. 8A-8F illustrate XPS spectra: XPS spectra of S 2p (FIGS. 8A-8C) and Mo 3d (FIGS. 8D-8F) spectra before (dashed trace) and after annealing (solid trace) for unfunctionalized metallic MoS₂ (FIGS. 8A and 8D) and functionalized (FIGS. 8B, 8C, 8E, and 8F) metallic MoS₂, according to some embodiments of the present disclosure. The nanosheets were annealed in a N₂ atmosphere for 24 hours at 150° C. The functional groups protected the MoS₂ from undergoing conversion from the metallic phase to the semiconducting phase. The Et₂NPh MoS₂ (FIGS. 8C and 8F) underwent some conversion to the semiconducting (2H) phase.

FIGS. 9A and 9B illustrate HER stability tests for Et₂NPh and metallic phase MoS₂: FIG. 9A illustrates LSV for glassy carbon electrodes deposited with Et₂NPh and metallic (1 T), according to some embodiments of the present disclosure. The scans were performed for as prepared electrodes and then electrodes following an anneal in a N₂ glovebox at 150° C. for 24 hours. The as prepared bulk (2H) MoS₂ electrodes are also shown for comparison. LSVs were taken at 5 mV/s in 0.5 M H₂SO₄ with an Ag/AgCl reference electrode and a vitreous carbon counter electrode. FIG. 9B illustrates the overpotential measured as a function of time to maintain a current density of 10 mA/cm² for the Et₂NPh and metallic (1 T) MoS₂ electrodes. Et₂NPh functionalized metallic MoS₂ outperformed the unfunctionalized metallic MoS₂ for H₂ generation within ˜7 min.

FIGS. 10A and 10B illustrate scanning electron microscopy images of (FIG. 10A) unfunctionalized metallic MoS₂, and (FIG. 10B) Et₂NPH functionalized metallic MoS₂, according to some embodiments of the present disclosure. The nanosheets are deposited on a Si wafer.

FIG. 11 illustrates fits to XPS spectra of unfunctionalized metallic MoS₂ (Panels A, G, and M), Et₂NPh functionalized metallic MoS₂ (Panels B, H, and N), OMePh functionalized metallic MoS₂ (Panels C, I, and O), BrPh functionalized metallic MoS₂ (Panels D, J, and P), Cl₂Ph functionalized metallic MoS₂ (Panels E, K, and Q), and NO₂Ph functionalized metallic MoS₂ (Panels F, L, and R). The S 2p spectra (Panels A-F) show differences between the metallic and functionalized MoS₂ due to a S—C bond. There is little difference in the Mo 3d spectra (Panels G-R) because the Mo does not participate in the bonding of the functional group. Panels G-L do not include MoO₂ in the fits while Panels M-R do include MoO₂ in the fits. The dashed traces are the residuals from the fits.

FIG. 12 illustrates S 2p_(3/2) peak positions for metallic Mo—S bonds (˜161.5 eV, squares) and S—C bonds (˜163 eV, circles) plotted against the Hammett parameter. Unfunctionalized MoS₂ was estimated to have a Hammett parameter of 0.23 from the DRIFTS data.

FIGS. 13A-13C illustrate XPS spectra unfunctionalized metallic MoS₂ nanosheets (FIG. 13A), OMePh functionalized metallic MoS₂ nanosheets (FIG. 13B), and BrPh functionalized metallic MoS₂ nanosheets (FIG. 13C) annealed in air at 150° C. for 24 hours, according to some embodiments of the present disclosure. The films decomposed into MoO₃.

FIGS. 14A-14F illustrate XPS spectra of S 2p (FIGS. 14A-14C) and Mo 3d (FIGS. 14D-14F) spectra before (dashed trace) and after annealing (solid trace) for BrPh functionalized MoS₂ nanosheets (FIGS. 14A and 14D), Cl₂Ph functionalized MoS₂ nanosheets (FIGS. 14B and 14E), and NO₂Ph functionalized MoS₂ nanosheets (FIGS. 14C and 14F), according to some embodiments of the present disclosure. The nanosheets were annealed in a N₂ atmosphere for 24 hours at 150° C. The functional groups protected the MoS₂ from undergoing conversion from the metallic to the semiconducting state.

FIG. 15 illustrates an optical microscopy image of monolayer MoS₂ flakes synthesized by chemical vapor deposition (CVD) with a characteristic length of up to about 50 m and having a thickness of about 0.8 nm, according to some embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that some embodiments as disclosed herein may prove useful in addressing other problems and deficiencies in a number of technical areas.

Therefore, the embodiments described herein should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.

The present disclosure relates to metal chalcogenides, for example molybdenum disulfide (MoS₂), functionalized with ligands, where further examples of metal chalcogenides include transition metal chalcogenides such as WSe₂, MoS₂, WS₂, MoSe₂, WTe₂, and MoTe₂. In some embodiments of the present disclosure, as shown herein, functionalizing nanosheets and/or nanoparticles of transition metal chalcogenides provides a synthetic chemical route for controlling the electronic properties and stability within traditionally thermally unstable metallic states. In some embodiments of the present disclosure, and as shown herein, the fundamental electronic properties of metallic (1 T phase) nanosheets of MoS₂ may be modified through functionalization with ligands by covalent bonds, resulting in the direct influence of the modified MoS₂ catalyst on the kinetics of the hydrogen evolution reaction (HER), surface energetics of the catalyst, and stability of the catalyst. Metal chalcogenides, as defined herein, have a composition defined by MXz, where z is between 1 and 3, inclusively. M may include at least one of Sc, Zr, Hf, Zr, Ti, Nb, Ta, V, Mo, Cr, Re, W, S, Pt, Fe, Cu, Sb, In, Zn, Cd, P, and/or Mn. X may include at least one of S, Se, and/or Te. Examples of metal chalcogenides include ScS₂, ScSe₂, SeTe₂, ZrS₂, ZrSe₂, HfS₂, HFSe₂, HfS₃, HfSe₃, ZrS₃, ZrSe₃, ZrTe₃, TiS₂, TiS₃, TiSe₃, NbS₂, NbSe₂, NbS₃, TaS₂, TaSe₂, TaS₃, TaSe₃, VS₂, VSe₂, MoReS₂, CrS₂, WSSe₂, MoSSe, MoWSe₂, MoTe₂, WTe₂, ReS₂, ReSe₂, ReNbS₂, ReNbSe₂, PtS₂, PtSe₂, PtTe₂, FeSe, CuS, CuSbS₂, CuInS₂, CuInSe₂, ZnS, ZnSe, CdS, CdSe, FePS₃, FePSe₃, MnPS₃, MnPSe₃, CdPS₃, and/or CdPSe₃. As shown herein, at least one of these metal chalcogenides may be functionalized with at least one ligand by a covalent bond. In some embodiments of the present disclosure, a metal chalcogenide that includes at least one of MoReS₂, CrS₂, WSSe₂, MoSSe, MoWSe₂, MoTe₂, and/or WTe₂ may be functionalized with at least one ligand by a covalent bond.

As used herein, the term “substantially” indicates a state and/or condition that is for the most part only one state and/or conditions. For example, a state and/or condition that is “substantially A”, may be 100% in state and/or condition A. However, a state and/or condition that is “substantially A” may contain some small amounts of B, for example within the limits of detection of the analytical method used to detect A, or within the limits of a separation method used to separate A from B. For example, a metal chalcogenide may be substantially in the 1 T metallic crystalline phase, meaning the metal chalcogenide may be 100% in the 1 T phase or at some value less than 100%; e.g. greater than 95%, greater than 99%, and/or greater than 99.9%.

In some embodiments of the present disclosure, chemically-exfoliated and/or CVD grown, metallic MoS₂ nanosheets may be functionalized with ligands containing electron donating and/or electron withdrawing groups, containing organic phenyl rings, where a phenyl ring is abbreviated herein as Ph. Functionalization of the metal chalcogenides, for example MoS₂, with ligands results in the ability to manipulate the electrochemical properties and stability of the metallic MoS₂. It was determined that MoS₂ functionalized with the most electron donating ligand, p-(CH₃CH₂)₂NPh, was the most efficient catalyst for HER of the ligands tested, with initial activity similar to the pristine metallic phase of MoS₂. The p-(CH₃CH₂)₂NPh-MoS₂ catalyst was shown to be more stable than unfunctionalized metallic MoS₂ and outperformed unfunctionalized metallic MoS₂ for continuous H₂ evolution within 10 minutes under the same conditions.

As shown herein, testing of MoS₂ as the metal chalcogenide in the form of a nanosheet, functionalized with various ligands, the overpotential and the Tafel slope for catalytic HER both correlated directly with the electron donating strength (Hammett parameter) of the pendant group on the ligand, in this case a phenyl ring. The results are consistent with a mechanism involving ground-state electron donation or withdrawal to/from the MoS₂ nanosheets, which modifies the electron transfer kinetics and catalytic activity of the MoS₂ nanosheets. In addition, the ligands tuned the workfunction of the metallic MoS₂ surface over a range of 800 mV. The workfunction correlated with HER activity, and the shallowest workfunction resulted in the highest activity for proton reduction. The ligands preserved the metallic feature of the MoS₂ nanosheets, inhibiting conversion to the thermodynamically stable semiconducting state (2H) when annealed at 150° C. for 24 hours in an inert atmosphere. This protection is critical to maintaining the catalytically active state of a metal chalcogenide catalyst; e.g. metallic MoS₂ nanosheets. Thus, without wishing to be bound by theory, it is proposed herein that the electron density and, therefore, reactivity of the metal chalcogenide HER catalysts may be controlled by functionalizing the catalysts with the appropriate electron withdrawing and/or electron donating ligands.

In some embodiments of the present disclosure, metallic MoS₂ nanosheets were modified with a series of substituted phenyldiazonium salts wherein the ligands (e.g. phenyl groups) formed S—C bonds between the ligands and the MoS₂ nanosheets. X-ray photoelectron spectroscopy (XPS) was used to verify the functionalization of the metallic MoS₂ and to measure the workfunction of the modified surfaces. HER studies were completed to determine the influence of the modified surfaces of the functionalized metal chalcogenide catalysts, e.g. functionalized MoS₂, on the HER kinetics. The stability of the functionalized metallic MoS₂, for both monolayer and multilayer sheets, was quantified by measuring the chemical environment (XPS) and HER activity before and after annealing (e.g. treating at elevated temperatures in a controlled N₂ environment); functionalization of the metallic sheets resulted in improved stability. The stability of both functionalized and unfunctionalized MoS₂ was further explored during continuous HER catalysis over a two-hour period of time. This work demonstrates that ligands provided increased stability for the catalytically active metallic phase of the MoS₂ nanosheets tested and that varying the ligands from electron withdrawing to electron donating groups directly influences the electronic properties and the HER catalytic reactivity of MoS₂ nanosheets.

Synthesis and Characterization:

Functionalized metallic MoS₂ nanosheets were synthesized following the procedure developed by Knirsch (ACS Nano 2015, 9, 6018). Briefly, bulk semiconducting MoS₂ was solution-exfoliated via intercalation with n-butyl lithium (n-BuLi) and a subsequent reaction with water produced MoS₂ nanosheets that were primarily in the metallic crystalline phase. After centrifugation/purification of these metallic nanosheets, they were reacted with a series of five diazonium salts (in separate reactions) to form the corresponding functionalized MoS₂ nanosheets. Throughout the remainder of this disclosure p-NO₂Ph-MoS₂, 3,5-Cl₂Ph-MoS₂, p-BrPh-MoS₂, p-OCH₃Ph-MoS₂, and p-(CH₃CH₂)₂NPh-MoS₂ are referred to as NO₂Ph, Cl₂Ph, BrPh, OMePh, and Et₂NPh, respectively, and the pristine metallic MoS₂ nanosheets as 1 T. Note that when appropriate, the as-purchased semiconducting crystalline phase of the MoS₂ powder is included for comparison and is referred to as bulk (2H). The five organic ligands tested each have a phenyl ring substituted with various moieties that are either electron donating or withdrawing (see FIG. 1A). This resulted in a range of electron donating/withdrawing properties of the ligands due to the ligands possessing a range of Hammett parameters, which are correlated to their dipole moments. The more negative the Hammett parameter, the more electron donating the moiety is to the phenyl ring and concomitantly to the MoS₂ surface. Et₂NPh possesses the most electron-donating moiety (e.g. Et₂N) and has a corresponding Hammett parameter of −0.43. Conversely, more positive Hammett parameters correspond to more electron withdrawing moieties, with the ligand NO₂Ph (having the NO₂ moiety) having the largest Hammett parameter of +0.78.

In some embodiments of the present disclosure, a nanocrystal and/or nanosheet composition may include a ligand covalently bound to a metal chalcogenide where the ligand may have at least one moiety such as at least one of a halogen, an amine, an amide, a ketone, thiol, and/or a nitro group. Examples of a halogen include fluorine, chlorine, bromine, iodine, and astatine. Examples of amines include at least one of methylamine, ethylamine, dimethylamine, and/or aniline. Examples of amides include at least one of acetamide, dimethylacetamide, a phosphonamide, and/or a sulfonamide. Examples of ketones include at least one of acetone, acetylacetone, and/or methyl ethyl ketone. Examples of thiol groups include at least one of ethanedithiol and/or methanethiol. Examples of nitro groups include at least one of nitromethane, nitrotoluene, and/or nitrobenzene. A moiety may be directly bound to at least one of the metal and/or to the chalcogenide of the metal chalcogenide. In some embodiments of the present disclosure, the ligand may include at least one intermediate group positioned between the moiety and the metal chalcogenide. Examples of an intermediate group include at least one of an aromatic group, a saturated hydrocarbon chain, and/or an unsaturated hydrocarbon chain. Examples of an aromatic group that include a phenyl ring include at least one of anthracene and/or bipyridine. Examples of other potentially suitable ligands include at least one of pyridine, pyrazine, imidazole, and/or anthracene.

The MoS₂ nanosheets were characterized with atomic force microscopy (AFM), scanning electron microscopy (SEM), and Raman spectroscopy. The metallic MoS₂ nanosheets were not monodisperse in lateral size or layer number. FIG. 1B illustrates AFM images of three MoS₂ nanosheets that were solution deposited onto a silicon substrate and are representative of the nanosheets described herein (e.g. having thicknesses between of 3 nm and 5 nm and characteristic lengths between 250 nm and 400 nm). The corresponding height profiles are shown in FIG. 1C.

In addition, FIG. 15 illustrates an optical microscopy image of monolayer MoS₂ flakes synthesized by chemical vapor deposition (CVD) with a characteristic length of up to about 50 m and having a thickness of about 0.8 nm, according to some embodiments of the present disclosure. To gain additional morphology information, metallic and Et₂NPh MoS₂ were measured with SEM (see FIGS. 10A and 10B); obvious changes to the morphology after functionalization are not evident. In addition, metallic and functionalized MoS₂ nanosheets were characterized with Raman, using an excitation wavelength of 633 nm (see FIG. 2A). All MoS₂ spectra contained the expected transitions at −383 cm⁻¹ (E¹ _(2g)) and ˜406 cm⁻¹ (A_(1g)). The Raman spectrum of the metallic MoS₂ has distinct features at 154, 230, and 326 cm⁻¹, which correspond to the J₁, J₂, and J₃ modes, respectively. The functionalized MoS₂ samples also have J₁ and J₂ peaks, whereas the J₃ peak is not observed with appreciable signal-to-noise, indicating that the functionalized MoS₂ mainly remained in the metallic phase.

The chemical environments of functionalized and bare MoS₂ were quantified by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), XPS, and combustion analysis. DRIFTS spectra (see FIG. 2B) confirmed the presence of the phenyl ring-containing ligands in all functionalized MoS₂ nanosheets except for the Et₂NPh ligand sample, which was not measured. The NO₂Ph ligand spectrum shows characteristic asymmetric (1518 cm⁻¹) and symmetric (1342 cm⁻¹) stretches for the NO₂ moiety, along with the NO₂ scissor mode at 853 cm⁻¹. The OMePh ligand spectrum shows symmetric (1463 cm⁻¹) and asymmetric (1440 cm⁻¹) stretches for the OMe moiety, along with Ph-O (1291 cm⁻¹ and 1250 cm⁻¹) and PhO—CH₃ (1025 cm⁻¹) stretches as well as the O—CH₃ rocking mode (1180 cm⁻¹).

The DRIFTS data (see FIG. 2B) also provide a measurement of the polar interaction between the MoS₂ and the phenyl ring of the ligands. The phenyl ring has four characteristic modes, split into two quadrant deformations around 1550-1600 cm⁻¹ and two semicircle deformations around 1450-1500 cm⁻¹. The higher-energy semicircle deformation around 1500 cm⁻¹ is only IR active when the phenyl ring has an electron donating group on it. This is consistent with the presence of a strong feature at 1492 cm⁻¹ in the OMePh ligand spectrum, but the lack of a higher energy peak in the BrPh ligand spectrum and the Cl₂Ph ligand spectrum (such a feature would likely be obscured by the NO₂ moiety asymmetric peak in the NO₂Ph ligand spectrum). In addition, the quadrant stretch modes are IR inactive for para-substituted phenyl rings where the substituents have identical electron donating or withdrawing character, maintaining a mirror of symmetry. Interestingly, the BrPh ligand spectrum shows almost no quadrant stretching, despite the fact that the para substituents are Br and MoS₂. This indicates that the MoS₂ contributes a similar amount of electron density to the phenyl ring as the Br group and has a similar Hammett parameter of around 0.23. In support of this analysis, the quadrant stretches on the other three phenyl groups behave as expected: the OMePh ligand spectrum shows the normal intensity ratio where the higher energy peak at 1594 cm⁻¹ is more intense than the peak at 1570 cm⁻¹; the Cl₂Ph ligand spectrum shows an inverted intensity ratio for its peaks due to extending resonance to the two Cl moieties (1562 cm⁻¹ peak is more intense than the 1591 cm⁻¹ peak); and the NO₂Ph ligand spectrum shows roughly equal intensity of the 1598 cm⁻¹ and 1576 cm⁻¹ peaks due to its less complete resonance with the phenyl ring.

XPS was utilized to determine the atomic compositions of the MoS₂, percentage of MoS₂ that was in the metallic phase, and the workfunction of the different films. The Mo 3d and S 2p core-level XPS results for the modified and unmodified MoS₂ are shown in FIGS. 3A and 3B. These XPS measurements on both the modified and unmodified MoS₂ nanosheets show that the concentration of C and O impurities was low. For each of the modified nanosheets, the atomic species expected for each moiety for each corresponding ligand was observed (N for Et₂NPh and NO₂Ph, Br for BrPh, and Cl for Cl₂Ph, data not shown). In the case of the OMePh ligand, there is not a unique atom, DRIFTS confirms functionalization for this functional group. Table 1 highlights the S:Mo and S:X ratio, where X is the unique atom for that functional group. The ratios were determined from multiple XPS measurements. The S:Mo ratios are close to the expected value of 2. The S:X ratios vary between samples due to differences in ligand coverage on the MoS₂ from batch to batch and the degree to which the ligand readily attaches to the MoS₂ nanosheets; therefore, only X was used as a qualitative confirmation of the presence of the ligands.

TABLE 1 The XPS atomic ratios (S:Mo and S:X), % of MoS₂ that is metallic (% 1T), and workfunction (ϕ) energies for the modified and unmodified metallic MoS₂ nanosheets. MoS₂ S:Mo S:X^(a) % 1T^(b) ϕ, eV^(c) 1T 2.0 NA 81 3.81 Et₂NPh 2.6 2.3 81 4.06 OMePh 2.2 NA 78 4.25 BrPh 2.1 2.4 82 4.75 Cl₂Ph 2.6 0.4 84 4.85 NO₂Ph 2.2 1.8 76 4.68 ^(a)X is the unique atom of the functional group, e.g. X = Cl for Cl₂Ph. ^(b)The % 1T is taken from Mo 3d fits, where MoO₂ is included. See SI for more details. Estimated standard deviation of +/− 8%. ^(c)The energy uncertainty for ϕ is +/− 25 meV.

In addition to the XPS results, elemental analysis measurements were completed to give relative quantities between Et₂NPh ligands and NO₂Ph ligands. For the CHN combustion analysis (see below), dried powders of the Et₂NPh and NO₂Ph MoS₂ nanosheets were used. This elemental analysis confirms the presence of N in both powders (see Table 2). However, the relative amount of CHN, per mole of MoS₂, was reduced in the Et₂NPh compared to the NO₂Ph powder. This result verifies a lower ligand coverage on MoS₂ for the Et₂NPh ligand compared to NO₂Ph ligand and is consistent with the XPS S 2p results (see FIG. 3C).

TABLE 2 CHN analysis of N containing functionalized MoS₂ Samples. Mass % Expected Found C H N C H N C H N NO₂Ph 35.87 2.00 8.38 6 4 1 6.00 3.98 1.20 Et₂NPh 18.02 2.20 2.55 10 14 1 10.00 14.52 1.21

In FIG. 3B, the XPS spectra of S 2p confirms the presence of S—C bonding for the functionalized MoS₂. The metallic MoS₂ spectrum clearly shows the S 2p spin orbit splitting of the metallic state (and a small contribution from the 2H state). The functionalized MoS₂ nanosheets have additional features at higher binding energy (162.5-165 eV), which are consistent with a S—C bond. The observation of this S—C bond gives additional evidence that the OMePh-MoS₂ sample is functionalized, since there is not a unique atom to identify the functional group with XPS. The functionalized MoS₂ nanosheets with electron withdrawing ligands (NO₂Ph, Cl₂Ph, and BrPh) have more intensity in the S—C region than the electron donating groups (see FIG. 3B). This suggests that the electron withdrawing groups have greater functional group coverage on the MoS₂ than the electron donating groups and is consistent with the CHN combustion analysis. Fits to the core level XPS spectra are shown in FIG. 11, Panels A-R. FIG. 3C plots the ratio of the areas of the 1 T Mo—S bond to the S—C bond. It is clear from FIG. 3C that the more electron withdrawing ligands have greater coverage on the MoS₂ than the more electron donating ligands. Mechanistically, the electron withdrawing ligands readily functionalize the negatively charged metallic MoS₂ nanosheets compared to the electron donating ligands; the degree of functionalization depends, at least partly, on the balance of nanosheet charge density and the ligands neutralizing the charge.

The S 2p peak positions of the S—C and metallic Mo—S bonds are consistent between the different functionalized nanosheets and do not have a dependence on Hammett parameter (see FIG. 12). High-resolution XPS core level measurements are sensitive to changes in the local atomic environment and are reported with respect to the Fermi level (i.e., electron binding energy). As expected, the functionalized MoS₂ nanosheets have an additional S peak because of the new bond formed between the MoS₂ and the ligand. This appears at higher binding energy with respect to the Mo—S bond. As for the S—C bond peak position, this does not change with ligand because the local atomic environment is the same (specifically, the S oxidation state is the same) and the Fermi level to core level energy difference is not changing with ligand.

The Mo 3d peaks (see FIG. 3A) also give detailed information about the chemical environment. Like S 2p, Mo 3d also has a spin orbit splitting (3.13 eV). The Mo 3d peak contains contributions from MoS₂, MoO₃, and MoO₂. For the S:Mo ratio, only the integrated Mo 3d area associated with MoS₂ was used (fits to the spectra are shown in FIG. 11, Panels A-R). From individual fits of the spectra, the percentage of MoS₂ in the metallic phase was quantified, which is ˜80% (see Table 1 above). Importantly, FIG. 3B demonstrates that the ligands do not change the Mo 3d features and the Mo environment remains the same. This again verifies that the ligands are bonding to the S and not to the Mo of the MoS₂ nanosheets.

Surface Energetics

To determine the degree to which the ligands influenced MoS₂ surface energetics, XPS was used to measure the workfunction (difference between the vacuum and Fermi levels). Table 1 above lists the average workfunction for the functionalized MoS₂ samples, where between 2 films and 5 films from separate reactions were measured for each ligand. There is a clear trend in how the workfunction varied with chemical functionalization (see FIG. 4); as the Hammett parameter increases (more electron withdrawing), the workfunction at the surface increases (requires more energy to remove an electron from the surface, deeper workfunction). Although there is some variation in the workfunction measurements of each sample type, due to differences in the amount of ligand per MoS₂, there is a positive correlation between the workfunction and Hammett parameter, with a large change of approximately 800 meV across the entire series. These differences in surface energetics in turn influence the HER activity of the MoS₂ nanosheets, which is discussed below.

HER Activity

To determine the effects of functionalization of metal chalcogenides on HER, drop-cast dispersions of modified MoS₂ onto freshly polished glassy carbon electrodes were performed to acquire linear sweep voltammograms (LSVs) (see FIGS. 5A and 5B). Typically, LSVs are used to determine the overpotential, Tafel slope, and current density of an electrochemical reaction of interest. The overpotential is the excess energy above the required thermodynamic potential in order to achieve a particular rate for hydrogen evolution. The Tafel slope is related to the kinetic rate and exchange current of the electrochemical reaction, and its slope can provide information on rate-determining steps for the reaction. With respect to the HER, it is desirable to minimize the Tafel slope, as this typically reduces the overpotential needed to reach appreciable catalytic current densities and is indicative of facile kinetics.

As the moiety on the phenyl ring was changed from the most electron withdrawing (Cl₂Ph, Hammett parameter=0.74) to the most electron donating (Et₂NPh, Hammett parameter=−0.43), a systematic shift to lower values in the overpotential and an increase in the catalytic rate towards hydrogen evolution was observed. As the Hammett parameter of the moiety decreased, the overpotential required to achieve 10 mA/cm² catalytic current density decreased from 881 mV for Cl₂Ph to 348 mV for Et₂NPh. This ˜500 mV shift was accompanied by a decrease in the Tafel slope from 213 mV/dec to 75 mV/dec. A tabulation of the electrochemical parameters can be found in Table 3. The Tafel slope of the most electron donating ligand (Et₂NPh) is similar to metallic MoS₂; however, the more electron withdrawing groups perform worse than bulk 2H (e.g. semiconductor phase) MoS₂. This correlation between the ligands and the MoS₂ catalyst HER activity gives insight into the mechanism by which the ligands and/or moieties interact with the MoS₂ surface.

TABLE 3 Electrochemical parameters with iR corrections for modified, metallic phase, and bulk semiconductor phase MoS₂ catalysts. The Tafel slope, exchange current density (j₀), and overpotential ( η) at 10 mA/cm² are listed for each of the electrodes.^(a) Tafel η, i = 10 Hammett Slope j₀ mA/cm² Parameter^(b) (mV/dec) (μA/cm²) (mV) Cl₂Ph 0.74 213 ± 33 1.5 ± 1.3 881 ± 13 BrPh 0.23 170 ± 7  0.4 ± 0.1 822 ± 11 OMePh −0.27 136 ± 8  1.2 ± 0.4 691 ± 23 Et₂NPh −0.43 75 ± 3 0.3 ± 0.1 348 ± 7  1T ~0.23^(c) 61 ± 7 1.9 ± 2.5 271 ± 36 Bulk (2H) NA 186 ± 54 1.1 ± 0.7 737 ± 46 ^(a)Averages and standard deviations are from three separate electrode preparations. ^(b)Values taken from Taft et al.²⁵ ^(c)Estimated equivalent value obtained from DRIFTS data.

FIG. 6 highlights the correlation between Hammett parameter and the activity of the functionalized MoS₂ catalysts. As the Hammett parameter is increased, so does the Tafel slope and the overpotential required to reach a catalytic current density of 10 mA/cm². NO₂Ph ligand data are not included in these graphs, as the nitro group was not stable under reductive conditions in acidic solution, presumably forming the ammonium substituted phenyl.

HER Mechanism

Based on the results described herein, and without wishing to be bound by theory, it is proposed herein that electron withdrawing (electrophilic) ligands remove more electron density from the metal chalcogenide nanosheets than the electron donating (nucleophilic) ligands, which ultimately determines the maximum amount of ligands obtainable per MoS₂ nanosheet. To further support this HER mechanism, electrochemical impedance spectroscopy (EIS) was completed to probe the electron transfer kinetics at the MoS₂/electrolyte interface, as the low frequency region in the EIS can be used to determine the charge transfer resistance of the interface. From the EIS data, the radius of the half circle was used to qualitatively describe the charge transfer resistance (Nyquist plots).

The EIS data (see FIG. 7) illustrate that the Et₂NPh electrode behaved similarly to that of metallic MoS₂. As the ligand became more electron withdrawing, the charge transfer resistance of the functionalized MoS₂ was larger than the bulk semiconducting phase of MoS₂. The EIS and HER electrochemistry results complement each other and support the mechanism that the functional group on metallic MoS₂ is either donating electron density or removing electron density from the metal chalcogenide nanosheet, which in turn changed the electron-transfer kinetics and HER activity. When excess charge was present on the MoS₂ nanosheets, the metallic phase maintained its high sheet conductance and low charge transfer resistance; conversely, when the excess charge was removed (by electron withdrawing ligands), the sheet conductance was reduced and the charge transfer resistance was increased, making the Cl₂Ph and BrPh MoS₂ behave worse than the bulk semiconducting phase for HER. Without wishing to be bound by theory, it is possible that the ligand may block the reactive S site, and as more ligands are present (electron withdrawing groups) the number of active sites may decrease. However, without being bound by theory, it is proposed that the amount of ligand attached to the MoS₂ nanosheets may be a balance of MoS₂ nanosheet charge and the Hammett parameter.

Stability

Although metallic MoS₂ is more catalytically active for HER than bulk semiconducting phase MoS₂, the metallic phase is thermodynamically unstable and with time and/or heat and tends to revert back to the semiconducting phase. Understanding how to preserve the metallic state of MoS₂ is important to maintaining the catalytic activity of the basal sites. To this end, modified and unmodified MoS₂ nanosheets, before and after annealing (e.g. heat treatment), were studied. When samples are annealed at 150° C. for 24 hours under atmospheric condition (e.g. at −21 mol % oxygen and −79 mol % nitrogen) the MoS₂ underwent conversion to MoO₃ and very little MoS₂ remained, as determined by XPS core level analysis (see FIGS. 13A-13C). It is evident from the spectra that metallic MoS₂ cannot withstand atmospheric conditions at elevated temperatures. However, when the MoS₂ nanosheets were annealed in a N₂ glovebox at 150° C. for 24 hours, the MoS₂ did not convert to MoO₃. FIGS. 8A-8F shows the XPS core level data of Mo 3d and S 2p before and after annealing for non-functionalized metallic MoS₂, OMePh functionalized MoS₂, and Et₂NPh functionalized MoS₂. The metallic, non-functionalized MoS₂ nanosheet converted from being predominantly in the metallic phase (˜80% 1 T) as determined by Mo 3d peak fitting) to a predominance of the semiconducting phase (˜75% 2H) under these conditions (see FIGS. 8A and 8D).

In comparison, the XPS data for the functionalized MoS₂ nanosheets did not show a change in the Mo 3d and S 2p peaks for OMePh, BrPh, Cl₂Ph, and NO₂Ph ligands (BrPh, Cl₂Ph, and NO₂Ph annealed spectra are similar to OMePh and are shown in FIG. 12). The spectra show no appreciable changes after annealing, indicating that the metallic nature of MoS₂ was preserved for the functionalized MoS₂, relative to unfunctionalized metallic MoS₂, for the annealing conditions tested. This result suggests that functionalization through a S—C bond and/or removing some amount of excess charge inhibits conversion back to the semiconducting state of MoS₂. The Et₂NPh functionalized MoS₂ nanosheets did change before and after annealing but to a much smaller degree than the pristine metallic MoS₂. This conversion may be due to excess charge remaining on the MoS₂ nanosheets following functionalization with the electron donating (nucleophilic) Et₂NPh ligand and/or relatively poor coverage of the MoS₂ nanosheet by the Et₂NPh ligand. The stability provided by functionalization to MoS₂ fits with our proposed mechanism by which the functional groups influence the HER mechanism. Specifically, the more electron withdrawing (electrophilic) ligands remove excess charge from the metal chalcogenide nanosheets, while “locking in” the metallic phase. When excess charge remains on the metal chalcogenide sheets, like with Et₂NPh ligands, the nanosheets have a lower thermodynamic barrier for the conversion of metallic phase (1 T) to the semiconducting phase (2H).

Two experiments were conducted to test the stability of the functionalized MoS₂ under HER conditions. First, the LSVs were measured for both “fresh” electrodes (consistent with those presented in FIG. 5) and electrodes that are annealed at 150° C. for 24 hours under N₂ environment (similar to conditions for the data presented in FIGS. 8A-8F). FIG. 9A illustrates the effect of annealing on HER performance and is shown for the Et₂NPh ligand-functionalized metallic MoS₂ and unfunctionalized metallic MoS₂. The unfunctionalized metallic MoS₂ degraded significantly and approached that of the bulk, semiconducting phase MoS₂ electrode. This result is consistent with the XPS annealing data (see FIGS. 8A and 8D). The Et₂NPh functionalized MoS₂ electrode was more resilient to this annealing step and degraded only slightly, which is again consistent with the XPS annealing data (see FIGS. 8C and 8F). The functional groups stabilize the underlying MoS₂, which prevents the conversion from the normally unstable metallic phase to the normally stable semiconducting phase.

Second, the durability of the Et₂NPh functionalized metallic MoS₂ and unfunctionalized metallic MoS₂ electrodes under HER conditions were studied over a two-hour period (see FIG. 9B), where the overpotential required to maintain 10 mA/cm² was monitored as a function of time. As can be seen in FIG. 9B, the unfunctionalized metallic MoS₂ performance degraded quickly and became worse than the Et₂NPh functionalized metallic MoS₂ electrode within 7 minutes. Over the two-hour period, the unfunctionalized metallic MoS₂ electrode required an additional 0.139 V to maintain the current density. This is very different from the Et₂NPh functionalized metallic MoS₂ electrode, which only degraded slightly over the two-hour period. The Et₂NPh functionalized metallic MoS₂ electrode only required an additional 0.014 V to maintain 10 mA/cm² over the two-hour period. The enhanced stability of the Et₂NPh functionalized metallic MoS₂ electrode, compared to the unfunctionalized metallic MoS₂ electrode, is very encouraging for utilizing these types of functionalized metal chalcogenide nanosheets for realistic long-term catalysis (e.g., HER) applications.

The results presented herein demonstrate a strong correlation between the electron donating strength of ligands and the surface energetics, electron transfer resistance, and the HER catalytic activity of functionalized MoS₂ nanosheets. The functionalized nanosheets are more stable to the thermally initiated phase transformation from the metallic 1 T phase to the semiconducting 2H phase. Furthermore, it is shown herein for an exemplary functionalized metal chalcogenide (Et₂NPh-MoS₂) that functionalization leads to better stability and long-term performance under HER conditions. These results provide a framework for understanding and controlling the balance between catalytic activity and stability for these unique 2D materials. Formation of S—C bonds via covalent surface functionalization protects the catalytically active, metastable, metallic phase. The HER catalytic activity is compromised for ligands that remove appreciable electron density from the MoS₂ nanosheets and have more functional groups per MoS₂ nanosheet. Thus, there may ultimately be a balance between catalytic activity (optimized initially for metallic phase MoS₂, relative to semiconducting phase and functionalized MoS₂) and stability (using a functional group that forms a S—C bond to kinetically protect the metastable metallic phase MoS₂).

MoS₂ Preparation and Functionalization

MoS₂ powder was obtained from Sigma-Aldrich and vacuum dried at 100° C. overnight prior to use. The chemically exfoliated metallic phase MoS₂ was prepared wherein 5 mL of n-BuLi (2.5 M) in hexanes was added to a suspension of 500 mg (3.1 mmol) of MoS₂ in 5 mL of dry hexanes and allowed to stir under an inert atmosphere for 48 hours. The reaction was then quenched with −100 mL of Milli-Q water. After hydrogen evolution ceased, the resulting suspension was then washed twice with −100 mL of hexane to remove organic impurities and then tip sonicated at ˜120 W for one hour in an ice bath. The solution was centrifuged at 800 rpm for 90 min to remove unreacted material. The solution was then decanted off and subjected to three centrifugations at 13200 rpm (21,400 g) for 90 minutes at 20° C. to remove small amounts of MoS₂ and LiOH. In addition to previous solution process, MoS₂ was synthesized by CVD, by using a three-temperature-zone furnace with dedicated temperature programs for sulfur flakes (Sigma Aldrich), MoO₃ powders (Sigma Aldrich), and sapphire substrates (University Wafer). The sulfur flakes and sapphire wafers were placed at Zone 1 and Zone 3, respectively. The MoO₃ powders were placed at Zone 2 but loaded in an insert tube which was hooked up to an individual flow controller. A gas of mixed argon and 4 vol % O₂ was supplied via the insert tube with a flow rate of 25 sccm while the mainstream flow rate outside the insert tube was about 125 sccm of pure argon. During the growth, the temperatures for Zones 1, 2, and 3 were 140° C., 530° C., and 850° C., respectively. The pressure of the growth chamber was maintained at 1 Torr. The growth duration was about 30 minutes. The given CVD grown method provides semiconducting MoS₂ monolayer sheets. To convert the semiconducting MoS₂ to metallic phase, the MoS₂ was soaked in 2.5 M n-BuLi/hexane solution in inert gas for 10 mins, following with twice hexane washing to remove excessive n-BuLi.

The MoS₂ was functionalized by suspending −100 mg of metallic MoS₂ or CVD sheets with substrates in Milli-Q water and adding dropwise −5 mL of a 10 mg/mL solution of the corresponding tetrafluoroborate salt: 4-p-Diazo-N,N-Diethylaniline Fluoborate (MP Biomedicals), 4-Methoxybenzenediazonium tetrafluoroborate (Sigma-Aldrich), 3,5-Dichlorophenyldiazonium tetrafluoroborate (Sigma-Aldrich), 4-Bromobenzenediazonium tetrafluoroborate (Sigma-Aldrich), or 4-Nitrobenzenediazonium tetrafluoroborate (Sigma-Aldrich). The solutions were then allowed to stir overnight. For MoS₂ nanosheets, the resulting precipitate was collected by filtration and washed twice with 20 mL of water to remove any unreacted material. The resulting material was then dried under vacuum.

Characterization

Metallic unfunctionalized MoS₂ nanosheets and functionalized metallic MoS₂ nanosheets were prepared for the various characterization experiments. The nanosheets were made by suspending metallic MoS₂ in DMF or suspending the modified MoS₂ in DMF or anisole and then drop-casting solutions of the modified and unmodified MoS₂ nanosheets onto different substrates (Si substrate—AFM and Raman, Au substrate—XPS, glassy carbon substrate—electrochemistry). All nanosheets were stored under a flowing N₂ environment (atmospheric pressure) or vacuum until being removed and exposed to ambient air for limited time. CHN analysis of the Et₂NPh and NO₂Ph powders were performed by Midwest MicroLab (Indianapolis, Ind.).

Photoelectron Spectroscopy

XPS data were obtained on a Physical Electronics 5600 system using Al K□ radiation. The XPS setup was calibrated with Au metal, which was cleaned via Ar-ion sputtering. The energy uncertainty for the core level data is +/−0.05 eV and for the workfunction measurements are +/−0.025 eV. In order to measure XPS on our series of MoS₂ nanomaterials, thin films were made on Au substrates by solution deposition. All samples were checked for and did not exhibit charging, which was verified by X-ray power dependence measurements. Atomic percentages have +/−5% error.

Electrochemistry

Electrochemical measurements were controlled by a CH Instruments 600D potentiostat coupled with a Pine analytical rotator. Measurements were taken in 0.5 M H₂SO₄ with a Ag/AgCl reference electrode and vitreous carbon counter electrode. For a typical measurement, 10 μL of a 1 mg/mL solution of the nanosheets suspended in DMF was drop cast onto a freshly polished 5 mm diameter glassy carbon electrode. All LSVs were performed at 5 mV/s and 1600 rpm and the electrolyte was degassed for 15 minutes with N₂ prior to experiments. Impedance measurements were carried out with the same setup with no rotation and measured at frequencies ranging from 1 GHz to 10 Hz at a constant overpotential of −0.29 V vs RHE.

Confocal Raman

An inVia Renishaw confocal Raman microscope with a Coherent HeNe 633 nm laser was used for characterizing the Raman signatures of MoS₂ nanosheet samples with and without functionalization. The samples were scanned by a 100× objective lens with 5% laser intensity (˜0.135 mW) and dispersed by 1800 lines mm⁻¹ in air under ambient conditions.

DRIFTS

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were acquired on a Bruker ALPHA FTIR Spectrometer using the DRIFTS sampling accessory. The samples were deposited by drop-casting onto aluminum-coated polished silicon wafer fragments (roughly 5 mm×5 mm), and the instrument was baselined against fragments from the same wafer. The instrument settings for both baseline and sample were 128 scans, 2 cm⁻¹ resolution, from 360 to 7000 cm⁻¹.

Atomic Force Microscopy

Atomic force microscopy (AFM) was used to image the 2D MoS₂ flakes that were solution deposited onto a silicon substrate. The ambient environment AFM uses a Park AFM XE-70 controller and is housed inside an acoustic box that is located on top of a vibration isolation table. Budget sensors silicon cantilevers (Tapp300G, ˜300 kHz) were used to image surface topography images in intermittent contact mode.

Scanning Electron Microscopy

Scanning electron microscopy (SEM) was used to image the MoS₂ nanosheets that were solution deposited onto a silicon substrate in a concentration that is consistent with the electrodes. All SEM images were taken in a FEI Nova 630 system.

The foregoing discussion and examples have been presented for purposes of illustration and description. The foregoing is not intended to limit the aspects, embodiments, or configurations to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the aspects, embodiments, or configurations are grouped together in one or more embodiments, configurations, or aspects for the purpose of streamlining the disclosure. The features of the aspects, embodiments, or configurations, may be combined in alternate aspects, embodiments, or configurations other than those discussed above. This method of disclosure is not to be interpreted as reflecting an intention that the aspects, embodiments, or configurations require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment, configuration, or aspect. While certain aspects of conventional technology have been discussed to facilitate disclosure of some embodiments of the present invention, the Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate aspect, embodiment, or configuration. 

What is claimed is:
 1. A composition comprising: a metal chalcogenide having a surface; and a ligand, wherein: the ligand is covalently bound to the surface.
 2. The composition of claim 1, wherein: the metal chalcogenide is defined by MXz, Z is between 1 and 3, inclusively, M comprises at least one of Sc, Zr, Hf, Zr, Ti, Nb, Ta, V, Mo, Cr, Re, W, S, Pt, Fe, Cu, Sb, In, Zn, Cd, P, or Mn, and X comprises at least one of S, Se, or Te.
 3. The composition of claim 2, wherein the metal chalcogenide comprises at least one of WTe₂, WSe₂, WS₂, MoS₂, MoSe₂, or MoTe₂.
 4. The composition of claim 3, wherein the metal chalcogenide comprises at least one of ScS₂, ScSe₂, SeTe₂, ZrS₂, ZrSe₂, HfS₂, HFSe₂, HfS₃, HfSe₃, ZrS₃, ZrSe₃, ZrTe₃, TiS₂, TiS₃, TiSe₃, NbS₂, NbSe₂, NbS₃, TaS₂, TaSe₂, TaS₃, TaSe₃, VS₂, VSe₂, MoReS₂, CrS₂, WSSe₂, MoSSe, MoWSe₂, MoTe₂, WTe₂, WS₂, MoS₂, MoSe₂, MoTe₂, ReS₂, ReSe₂, ReNbS₂, ReNbSe₂, PtS₂, PtSe₂, PtTe₂, FeSe, CuS, CuSbS₂, CuInS₂, CuInSe₂, ZnS, ZnSe, CdS, CdSe, FePS₃, FePSe₃, MnPS₃, MnPSe₃, CdPS₃, or CdPSe₃.
 5. The composition of claim 1, wherein the metal chalcogenide is in a form comprising at least one of a sheet or a particle.
 6. The composition of claim 5, wherein the sheet comprises at least one monolayer of the metal chalcogenide.
 7. The composition of claim 5, wherein the sheet has a thickness between 5.0 nm and 30 nm.
 8. The composition of claim 5, wherein the particle has a characteristic length between 5.0 nm and 50,000 nm.
 9. The composition of claim 1, wherein the ligand comprises at least one of an electron donating functional group or an electron withdrawing functional group, as measured by at least one of a Hammett parameter or a workfunction.
 10. The composition of claim 1, wherein the ligand comprises a moiety comprising at least one of a halogen, an amine, an amide, a ketone, thiol, or a nitro group.
 11. The composition of claim 10, wherein the ligand further comprises an intermediate group comprising at least one of an aromatic group, a saturated hydrocarbon chain, or an unsaturated hydrocarbon chain.
 12. The composition of claim 11, wherein the aromatic group is a phenyl ring (Ph).
 13. The composition of claim 12, wherein: the ligand comprises at least one of NO₂Ph, ClPh, BrPh, OCH₃Ph, or (CH₃CH₂)₂NPh, and the moiety is in at least one of the ortho position, the meta position, or the para position of the phenyl ring.
 14. The composition of claim 13, wherein the ligand comprises at least one of p-NO₂Ph, 3,5-Cl₂Ph, p-BrPh, p-OCH₃Ph, or p-(CH₃CH₂)₂NPh
 15. The composition of claim 1, wherein the metal chalcogenide is in a substantially crystalline phase.
 16. The composition of claim 15, wherein at least a portion of the crystalline phase comprises a 1 T metallic phase.
 17. The composition of claim 9, wherein the Hammett parameter is between −0.5 and 1.0.
 18. The composition of claim 9, wherein the workfunction is between 3.0 eV and 6.0 eV.
 19. The composition of claim 1, wherein: the composition catalyzes the hydrogen evolution reaction (HER), and the composition generates a HER catalytic current density of at least 10 mA/cm² when provided with an overpotential of less than 1000 mV.
 20. A composition comprising: MoS₂ in a form of at least one of a particle or a sheet; and a ligand covalently bound to the MoS₂, wherein: the ligand comprises at least one of p-NO₂Ph-, 3,5-Cl₂Ph-, p-BrPh-, p-OCH₃Ph-, and p-(CH₃CH₂)₂NPh-, the MoS₂ is it least partially in a 1 T crystalline phase and maintains the 1 T crystalline phase for at least two hours, the form has a characteristic length between 5.0 nm and 50,000 nm, the composition is characterized by a Hammett parameter between −0.5 and 1.0, the composition catalyzes the hydrogen evolution reaction (HER), and the composition generates a HER catalytic current density of about 10 mA/cm² when provided with an overpotential of less than 1000 mV. 